The Petrograd Soviet, or Petrograd Soviet of Workers' and Soldiers' Deputies, was the council set up in Petrograd (Saint Petersburg, Russia) in March 1917 as the representative body of the city's workers.
A workers' soviet had been created in St. Petersburg in 1905, see St. Petersburg Soviet. But the precursor to the 1917 Soviet was the Central Workers' Group (Tsentral'naya Rabochaya Grupa — Центральная Рабочая Група), founded in November 1915 by the Mensheviks to sit between workers and the new Central Military-Industrial Committee in Petrograd. The group became increasingly radical as the war progressed and the economic situation became worse – encouraging street demonstrations and issuing 'revolutionary' proclamations.

The Ispolkom
Existing as an alternate source of authority to Prince Lvov's Provisional Government created a situation described as dvoevlastie (dyarchy or dual power) which lasted from March until the October Revolution. The Ispolkom often publicly attacked the 'bourgeois' Government and boasted of its de facto power over de jure authority (to use a later quote from Trotsky) – it had control over post and telegraphs, the press, railroads, food supply, etc. A 'shadow government' with a Contact Commission (created March 8) to "inform... [the Provisional Government] about the demands of the revolutionary people, to exert pressure on the government to satisfy all these demands, and to exercise uninterrupted control over their implementation." On March 19 the control extended into the military front-lines with commissars appointed, with Ministry of War support.
The Ispolkom expanded to nineteen members on April 8, nine representing the Soldiers' Section and ten the Workers' Section. All members were socialists, the majority Mensheviks or SRs, there were no Bolshevik. After the All-Russian Consultation of Soviets, the Petrograd Soviet began adding representatives from other parts of the nation and the front-lines, renaming itself the All-Russian Soviet of Workers' and Soldiers' Deputies. The committee became the All-Russian Central Executive Committee (CEC or VTsIK) with over seventy members – including no peasant representatives. The mass meeting of the entire body were tapered off, being reduced from daily in the first weeks to roughly weekly by April.

Riots and street protests
The Bolsheviks rapidly assumed the mantle of the 'official' opposition, while the socialist groups in cabinet could now be attacked for the failures of the Provisional Government. The Bolsheviks began a strong run of propaganda – in June 100,000 copies of Pravda (including Soldatskaya Pravda, Golos Pravdy, and Okopnaya Pravda) were printed daily. In July over 350,000 leaflets were distributed. The Bolsheviks attempted another uprising on July 3–July 5 – a further wave of riots without success.
The rise of Kerensky, and the later shock of the Kornilov Affair, polarized the political scene. The Petrograd Soviet moved steadily leftwards just as those of the centre and right consolidated around Kerensky. Despite the events in July the Ispolkom moved to protect the Bolsheviks from serious consequences, adopting resolutions on August 4 and August 18 against the arrest and prosecution of Bolsheviks. Still leery of the Ispolkom the government released many senior Bolsheviks on bail or promise of good behaviour.
In the August 20 municipal elections the Bolsheviks took a third of the votes, a fifty percent increase in three months. There was also a general falling away in the attendance of soviet meetings, indeed many of the smaller soviets no longer existed except on paper.
During the Kornilov Affair the Ispolkom was forced to use the Bolshevik's Military Organization as its main force against the "counter-revolution." Kerensky ordered the distribution of 40,000 rifles to the workers of Petrograd, many of which ended in the hands of Bolshevik groups.
As other socialist parties abandoned the Soviet organizations, the Bolsheviks increased their presence. On September 25 they gained a majority in the Workers' Section and Leon Trotsky was elected chairman. He directed the transformation of the Soviet into an adjunct of the party, bypassing the Menshevik-SR Ispolkom and non-Bolshevik soviets to form a new Bolshevik control structure.
The Bolsheviks used their power in the Petrograd Soviet to set-up a 2nd All-Russian Congress of Soviets on October 20 (agreed September 26), despite only eight of 169 soldiers' or workers' soviets expressing support. With elections to the Constituent Assembly looming the Bolsheviks had to use their power quickly to discredit the elections. The Ispolkom denounced the Congress and the steps the Bolsheviks were taking to create its delegates. Suddenly and without reason, on October 17, the Ispolkom Bureau approved the Congress.